Delaying the birth of premature babies by just a few days can prevent brain damage and disability, researchers said on Friday.

Some obstetricians believe babies who are not thriving in the womb should be born as soon as possible, but new research suggests it may be better to wait.

“On average hanging on a bit longer improves brain development,” Professor Jim Thornton, of the University of Nottingham in England, said in an interview.

In a study of 588 premature babies in 13 European countries, Thornton and his colleagues looked at the rate of disability among children who were born as soon as a problem was identified and those whose birth was postponed by an average of 4.5 days.

There was no difference in the death rates between the two groups but the disability rate at two years old was eight percent among babies born immediately and four percent in the group that waited.

The biggest difference in disability between the two groups was in infants born at less than 31 weeks into a pregnancy.

All of the women in the eight-year study were between 24 and 36 weeks into their pregnancy and tests confirmed they were having problems.

“A lot of babies are delivered prematurely because the obstetrician says it would be safer out (of the womb) than in,” said Thornton, who reported the research in The Lancet medical journal.

“Hopefully our results will give doctors better information to make those crucial decisions,” he said.

About 90 percent of babies born at 28 weeks gestation survive, he added.

Deciding the optimum time of delivery for a baby that is not thriving in the womb can be a difficult decision for doctors because premature babies have a higher risk of suffering from respiratory ailments, cerebral palsy and other problems.

But if delivery is delayed too long here is a risk of stillbirth.

“Assessing the well-being of the fetus remains one of the most vexing challenges in obstetrics,” said David Grimes of public health organization Family Health International.

But he added the research “will help ensure that fetuses in peril are born at the optimum time.”